1. Field of the Invention
The present invention relates to an access unit used for branching ports of a concentrator forming a local area network (LAN) with loop topology in conjunction with a plurality of workstations such that desired number of workstations can be additionally connected, and a concentrator system using the access unit.
2. Prior Art
FIG. 7 shows a conventional LAN comprising a conventional access unit. In the conventional LAN of FIG. 7, a concentrator S comprises four ports P one or all of which a workstation W is connected to in order to form a local area network with loop topology (LAN). Connectors C.sub.1 and C.sub.2 are provided for coupling another concentrator.
A relay R is incorporated within each of the ports P for the workstation W. When the workstation W is connected to the port P, the relay R is switched to open the circuit and switch the circuit to the workstation W side. When the workstation W is removed, the circuit is closed. Therefore, the workstation W is inserted and removed from the LAN within the limitation of the number of the ports P of the concentrator S.
In case where the workstation W is positioned at a location distant from the location where the concentrator S is installed, it is troublesome to provide the cable. When the number of the workstations W is large, the wiring becomes complex.
Since the number of the ports P in the concentrator S is limited, it is difficult to expand the number of the workstations W over the number of the ports P. Thus, it may be difficult to sufficiently meet with the requirement when the LAN is to be expanded. Conventionally, an access unit au as shown in FIG. 7 is used. The access unit is a kind of branching device conneted to one of the ports P in the concentrator S. The access unit au comprises a connector co connected to the port P in the concentrator S and a plurality (three in FIG. 6) of port connectors cp for the workstation W. The provision of the access unit au enables the number of the workstations W connectd to the concentrator S to increase in order to expand the LAN. Further, a single cable can be enough from the access unit au to the concentrator S, so that it is unnecessary to couple the workstations W individually to the concentrator S, which is not troublesome.
However, in the conventional access unit au, the number of the ports P is limited to a predetermined number, for example, 4, it is not easily possible to freely alter the number of the branching ports which is not able to expansion.
Conventionally, even if a single workstation W is added, the access unit au having a plurality of (normally, four) ports is used, which is not cost-effective. In addition, in the port P of the concentrator S connected to the access unit au, the circuit of the concentrator S must be open and connected to the circuit of the access unit au. In short, the circuits need to be switched over the two stages. Conventionally, it has been proposed that power energy is supplied from the connecting workstation W to the access unit au for supporting circuit switching. However, this system needs individual power lines to the workstation W and the access unit au, respectively. Then, the structures of the cables and the connectors are different from the general types for data transmission, which is not in practise.